O'Brien is best remembered for kicking the game-winning field goal in the final seconds of Super Bowl V, making him the first of only three placekickers to accomplish such a feat.

[1] O'Brien enrolled at the United States Air Force Academy in 1966, but received a medical discharge for ulcers.

In 1968, O'Brien led the nation in scoring with 142 points, with 44 receptions for 1,107 yards and 12 touchdowns as a receiver and making 13 field goals and 31 PATs as a placekicker.

[4][5] The Colts ended the 1970 regular season with an 11–2–1 record and defeated the Cincinnati Bengals and Oakland Raiders in the playoffs to earn a spot in the Super Bowl.

The Cowboys took an early lead on two field goals by Mike Clark before Colts quarterback Johnny Unitas completed a tipped pass to tight end John Mackey for a 75-yard touchdown.

Later in the second quarter, the Cowboys scored a touchdown on a Craig Morton throw to Duane Thomas to take a 13–6 lead.

Colts running back Tom Nowatzke scored two plays later, capped by an O'Brien PAT to tie the game again at 13–13.

An offensive holding penalty followed by a Morton interception to linebacker Mike Curtis gave the Colts the ball back on the Cowboys' 28-yard line.

This feat has since been accomplished three more times, twice by Adam Vinatieri with the New England Patriots and once by Harrison Butker with the Kansas City Chiefs.

Citing his declining field goal accuracy, Colts head coach Howard Schnellenberger and general manager Joe Thomas asked O'Brien to remain on the team as a wide receiver.

[10] O'Brien spent the offseason competing for the kicking position before he was cut in September 1975 prior to the start of the regular season.